The Ruler and the EggThe things we do when learning the craftAt the end of 2000, I took my very first official photography class at North Central College in Naperville, IL. The class was led by Kirk Kreutzig, a successful photojournalist who holds several patents on color correcting filters. You can read about him in this article which appeared in the Chicago Tribune the year before about his underwater lenses: http://articles.chicagotribune.com/1999-10-12/news/9910120230_1_filters-steel-industry-wild-kingdom
It was a community college adult education class, so it wasn't very intense, but he was very instrumental in lifting the fog that surrounded exposure. I had recently read the Ansel Adams book series on photography. I didn't get much out of them as it was geared toward a different time on different equipment. All the information was there, but I think hearing it straight from Ansel Adams would have been helpful. But since he was dead almost 17 years, that wasn't going to happen. So, enter Mr. Kreutzig. The Egg ExperimentSo, what does the camera "see" when you point it at something? This is the first step in understanding exposure. This applies to spot metering and center-weighted metering for the most part. Most cameras now have some sort of evaluative metering which will take all levels of light into consideration when guessing the exposure. It's a better guess, but it's still a guess. The camera evaluates everything you point it at as 12% grey. Yes, I know, you've always been told it's 18% grey, but see evidence to the contrary here. The first experiment we did to illustrate this example was the egg test. Place and egg on a white surface with a white background with as little variation in shadows as possible. Everything needs to be white. Set the camera to use spot metering an meter off the whitest part. Use aperture mode wide open (lowest f-stop). Most important, do not change the suggested exposure the camera gives you. With your f-stop fixed, the shutter speed will be set to what the camera thinks is the correct length of time to let light fall on the sensor. Take a few exposures. If printing, instruct the printer NOT to adjust the exposure to compensate, because your local Walgreens or Walmart will color correct by default if you don't tell them not to. (I did this back in 2000 we were all still using film cameras. I know, only 15 years ago. Seems like forever.) If you do this test with a digital camera, make sure your camera is not set to auto-adjust ISO. Shot #1, ISO 400, f4.8 at 62mm. Suggested exposure is 1/20sec using the camera's metering gauge As you can see, without any adjustments, the whole scene is too dark. The histogram shows most of the values to the left side of center, which is a shadow area. If you take a photo of an "18%" grey card, you will see a similar histogram, except there will be a spike at one point since most of the values should be middle grey. Simple exposure compensation corrects for this. But setting the camera to EV+1, we get closer. Suggested exposure is 1/8sec. (Which actually ends up being EV+1.5) What we want is the right side of the histogram curve as close to the right side as possible without clipping the highlights. You will assume correctly that this works when taking a photo of a very dark subject. In that case, EV-1 or more will help keep the darks dark without overexposing the bright values. But we are still well off the highlight area of the histogram. We need to go further. I then went to EV+2 to see what effect that would have. Suggested exposure is 1/4sec (1 additional stop) Closer, but still not white! I actually had to go to EV+3. Suggested exposure 1/3, which is actually 1/2 stop more. I think any more and it would have been too white. You never want to go too far with the compensation or else you start clipping highlights. Best to stay a bit back so you have room to adjust in either direction. This is an extreme example, however. Taking a photo of a snowy scene is different and probably would not require such a large EV compensation. Start with EV+1 or +2 and you will usually be safe. If your camera has a "snow or sand" setting, it will usually do this compensation for you. So great, you now have a perfectly exposed photo of an egg at f4.8 (f1.4 or 2.8 if you have a fast lens). Let's assume ISO is 400 and shutter speed is 1/3sec. You now have the three elements needed to be creative. Also remember that "moving +/-1 stop" is the same as either cutting the light hitting the sensor in half (stopping down) or doubling the light hitting the sensor (opening up) Using the exposure above at ISO400, f4.8, 1/3sec, we can easily compute all other combinations without doing any math. Let's say you want to get more sharpness front to back in your snow image. If you have elements in the background, f4.8 won't give you the background sharp, so you need to stop down (raise your f-stop) to narrow the aperture. If you look at the chart below, it's pretty simple. It's in 1/2-stop increments for simplicity.
To sharpen the background (more Depth of Field or DOF), you could go to f11, and how ever many steps take you to f11, you slow down your shutter speed by the same number. In this case, 5 steps on the aperture dial gets to f11. 5 steps slower on the shutter speed gets to 2sec. 2sec at f11 will give you the same exposure as 1/3sec at f4.8 (or 1/8sec at f2.8) If you want f8, 3 steps on both. If the ISO changes, it affects both the shutter speed and aperture. If you change the above exposure, which was at ISO400, to ISO200 (stopped down one stop) we've effectively "simulated" cutting the light hitting the sensor in half. I say "simulated" because you are not changing the amount of light hitting the sensor, you are simply changing how the sensor reacts to the light hitting it. ISO400 amplifies the light hitting it so you can use a smaller f-stop or faster shutter speed. By going to ISO200, we've cut down on the sensor's need to amplify the light, so we have to compensate by adding more of it. To match the 1 stop change in ISO, we have to change the combination of aperture and shutter speed one stop down on either OR a combination of aperture AND shutter speed (1/2 stop down and less light on both).
This is especially useful at night if you want to take moonlight landscapes. You can set your ISO to the highest it will go and take some test shots at different shutter speeds. Once you find the right shutter speed at the high ISO, you can back down the ISO and know exactly how much to increase the shutter speed to match that exposure at an ISO that will not produce as much noise. The Ruler Experiment (Depth of Field)The other experiment we did was using a ruler. We were to set up the ruler on edge with the camera set at one end pointing down the length of it. The camera ISO should be fixed and the f-stop should be at it's widest. The shutter can be set at what gets the correct exposure. The focal point, however should be at the very closest setting the lens can manage.
The lines on the ruler are a great example of the extremely tight depth of field the camera has in this configuration. The enlargement on the right shows relative sharpness of just one inch (12 to 13). But since the ruler is on an angle here, the actual DOF is actually a little less. This is why many close-up photos of flowers fail to bring all parts of the flower into focus. Many times the photo is being taken at very close range to fill the frame and at a wide aperture to let the most light in so the shutter speed can be set fast to stop any motion in the flower. In order to increase DOF, you have to stop down the aperture (smaller circle), but in doing so, you have to open up (slow) your shutter speed to let more light in. But you also have ISO to save the day. By increasing your ISO, you can keep your shutter speed where you want it, but at the cost of more noise in your photos. If you have a full-frame DSLR, you do not have as much of a problem however since the pixels are larger and can handle the amplification better.
At f11, things get a little clearer. The photo on the right shows sharpness from 10 3/4" to 12 1/2" (1 3/4") with some unsharpness on either side of that, but the numbers to the back are still readable.
At f22, the depth of field increases dramatically, from before the 9 inch mark to the back. All numbers appear readable on the full size image. When we extend the zoom out to the maximum of 250mm, things get funky. At f6.7, acceptable focus is roughly nil, although it appears like 12 1/4" - 12 3/8" is acceptable, which is less than 1/8". But, I can't even get any DOF calculators to give me anything more than 1mm! So, remember, at high telephoto settings and closest focus, we have an extremely narrow DOF.
At f11, we improve slightly to 12 1/8" - 12 1/2", or 3/8" total apparent focus. The DOF calculator still shows less than 1mm!
At f22, we've only improved to maybe 5/8" total apoparent focus. And with the DOF calculator, I did get close to .02". That's 2 HUNDRETHS of an inch!
So try these experiments yourself. The pixels are free and it's a great way to get a feel for your meter and lens. Hopefully it will help you make good decisions when you want to photograph something that you want to be all sharp and in focus. Or maybe it will help you be creative and throw everything out of focus. Whatever you want to do, make sure you put the egg back in the refrigerator or someone might get a big surprise when they sit down. What did we learn today?
Now, go expose yourself. -Jim P.S. If you liked this article, please comment or "Pin it" or "Like it" or do whatever you kids do with this stuff today
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